The present invention relates to a booster circuit for a pre-drive circuit of a brushless direct current single-phase motor. The brushless direct current single-phase motor is ideal as a fan motor that exhausts heat generated within the housing of an electronic instrument to the exterior.
In a conventional electronic instrument, a plurality of electronic components are accommodated in a relatively narrow housing as office automation equipment, such as personal computers and copiers. The heat generated from the electronic components is confined within the housing and there is a danger of thermal breakdown of the electronic components.
Therefore, vent holes are provided in the walls, for example, of the housings of such electronic components. An air blower (fan motor) is attached near the vent holes to exhaust heat from inside the housing to the exterior.
It is not uncommon for brushless direct current single-phase motors to be used for this type of fan motor. The drive circuit of this type of brushless direct current single-phase motor will be explained in reference to FIG. 4.
In FIG. 4, the drive circuit 31 is configured with four main switching elements, which are N-channel MOS-type power FET (field effect transistors) PF1 to PF4, and a diode D31.
The four power field effect transistors (FETs) PF1 to PF4 (hereinafter PF1, PF2, PF3 and PF4) are divided into two pairs of serially connected power FETs or, in other words, a series connection resulting from PF1 and PF3 and a series connection resulting from PF2 and PF4. They are respectively connected between the power supply +B and ground and have polarities as shown in the drawing. Diode D31 is connected in the forward direction with respect to the power supply +B between the power supply +B and the two pairs of serially connected power FETs (PF1, PF3, PF2, PF4). The coil (motor coil) L1 that is subject to driving is connected between the junction of PF1 and PF3 and the junction of PF2 and PF4.
The pre-drive circuit 32 is a circuit that receives signals from the motor rotation position detector 33 and the duty ratio setting instrument 34 in pulse width modulation (PWM). The pre-drive circuit 32 also supplies gate signals GS1 to GS4 to PF1 to PF4 according to the appropriately set duty ratio. The pre-drive circuit 32 performs ON/OFF control of the power FETs PF1 to PF4.
The coil L1 is provided on the motor stator (not shown in the drawing), and connected as shown. Current flows from the left end in the drawing to the right end or from the right end to the left end at the prescribed ON/OFF timing according to PF1 to PF4. Thus, a dynamic magnetic field (rotating magnetic field) is created.
A permanent magnet is provided on the motor rotor (not shown in the drawing). The rotor rotates by means of the permanent magnet following the dynamic magnetic field.
The pre-drive circuit 32 is equipped with gate circuit portions 32a to 32d that individually output gate signals GS1 to GS4. In this case, power supply +Vp, resulting from direct current power supply +B being boosted by a booster circuit 35 is provided to the gate circuit portions 32a, 32b that output gate signals GS1 and GS2 to PF1 and PF2. Power supply +Vp is provided to the gate circuit portions 32c, 32d that output gate signals GS3 and GS4 to PF3 and PF4 without direct current power supply +B being boosted.
This is such that PF3 and PF4 turn ON if the gate (control input terminal) is slightly higher than the ground potential since the source is grounded. On the other hand, PF1 and PF2 are at the power supply +B side flanking coil L1. For this reason, in the normal case in which the drive voltage of the coil L1 is nearly equal to power supply voltage +B [V], PF1 and PF2 cannot be turned ON. This occurs if the power supply +Vp [V] resulting from adding the voltage between the gate and the source required for turning PF1 and PF2 ON to a voltage greater than the power supply voltage, specifically to the power supply voltage +B [V], is not provided to the gate.
By using the booster circuit 35, it is possible to boost the voltage +B [V] of the direct current power supply +B to the prescribed voltage +Vp [V]. The prescribed voltage +Vp can be provided to the gate circuit portions 32a, 32b. The levels of the gate signals GS1 and GS2 of PF1 and PF2 can be changed to a voltage that is higher than that of gate signals GS3 and GS4 of PF3 and PF4. In addition, PF1 and PF2 ON/OFF control become possible.
FIG. 5 shows the configuration of a conventional booster circuit along with the drive circuit 31 in FIG. 4.
As shown in FIG. 5, the booster circuit consists of a charge pump circuit that is equipped with diodes D41, D42, capacitors C41, C42 and resistors R41, R42 having polarities as in the drawing.
In the drawing, +Vp1 becomes the power supply to gate circuit portion 32a of the pre-drive circuit 32 shown in FIG. 4, and +Vp2 becomes the power supply to gate circuit portion 32b of the same pre-drive circuit 32.
FIG. 6(a) is a voltage waveform drawing of power supply +Vp1 in FIG. 5, in other words, of gate signal GS1. FIG. 6(b) is a voltage waveform drawing of gate signal GS4.
The voltage waveforms of the power supply +Vp2 (gate signal GS2) and gate signal GS3 are also the same as the FIGS. 6(a) and 6(b), with the exception of the phases being different. Note that FIGS. 6(a) and 6(b) give examples of the case in which the duty ratio is 100%.
Through this type of booster circuit, it becomes possible to change the level of gate signal GS1 (GS2) to a voltage that is higher than that of gate signal GS4 (GS3), and ON/OFF control of PF1 (PF2) becomes possible.
However, in the above conventional circuit, the power supply +Vp1 (+Vp2) is generated by capacitor C41 and resistor R41 (capacitor C42 and resistor R42), and gate signal GS1 is created using this power supply +Vp1. For this reason, the voltage waveform of gate signal GS1 is such that rounding occurred during rise or fall compared to the squares shown by the dashed line in FIG. 6(a), and operation of the motor became extremely unstable.
Therefore, one solution is to have booster circuits that use integrated circuits (ICs), transformers and other elements to obtain a stable high voltage and to operate the motor stably. However, these booster circuits had problems in that the transformers and the ICs, in particular, are expensive. Also, the space occupied by the transformer is large and packaging to a small printed wiring board that is built into the motor is difficult.